The present invention relates to systems, apparatus, and/or methods for use in the processing of metal bearing material (e.g., the reduction of iron bearing materials such as iron oxide using a direct reduction process).
Hearth furnaces have been manufactured for decades and present a proven technology for various purposes, including reduction of metal bearing materials. Such furnaces have been widely used in the mineral industry for drying, preheating, roasting, calcining, steel plant waste treatment, iron ore reduction, and production of metallic iron nuggets. A process to produce direct reduced iron (DRI) may involve the following generalized processing steps: feed preparation, drying, furnace charging, preheating, reduction, cooling, product discharge, and product passivation. A process to produce metallic iron nuggets may involve all of the steps for producing direct reduced iron plus a high temperature step in which the metallic iron formed is fused to form metallic iron nuggets, and the associated slag melts and segregates from the iron. In addition, a physical separation step is generally required to separate the metallic iron nuggets from the slag and furnace hearth layer after the products have cooled and solidified.
Various issues related to the design of such furnaces (e.g., those used to produce DRI or metallic iron nuggets) include, but are clearly not limited to, material handling, engineering/construction, maintenance, flue gas treatment to remove particulates and recover sensible heat, and in some cases provide it as make-up gas, hearth integrity, and overall system reliability.
One type of hearth furnace, referred to as a rotary hearth furnace (RHF) has been adapted for the production of DRI and metallic iron nuggets. Several rotary hearth furnaces have been built for DRI production. For example, one such RHF is used in the FASTMET process developed by Midrex Corporation and is described in the article “Development of the FASTMET as a New Direct Reduction Process,” by Miyagawa et al., 1998 ICSTI/IRONMAKING Conference Proceedings.
The RHF has also been used to produce metallic iron nuggets. For example, such processes include the ITmk3 process described in U.S. Pat. No. 6,036,744, to Negami et al., entitled “Method and apparatus for making metallic iron,” and also the QIP process, described in the article “New coal-based process, Hi-QIP, to produce high quality DRI for the EAF,” by Sawa et al., ISIJ International, Vol. 41 (2001).
Processing in a typical RHF operation may include forming balls, briquets, or similar agglomerates composed of a mixture of iron ore, reductants (e.g., coal anthracite, coke, etc.), various slagging constituents (e.g., lime hydrate, fluorspar, soda-ash, etc.), water, and binders (e.g., bentonite or lime hydrate). The agglomerates may be dried in a separate drying oven and charged to the hearth of the furnace in a charging zone thereof, or perhaps, wet agglomerates may be charged directly to the hearth of the furnace in the charging zone.
The hearth is rotated to carry the agglomerates from the charging zone into a preheat zone of the RHF where the temperature is increased so as to drive off most of the volatile matter from the coal and other additives. Further rotation of the hearth carries the agglomerates into a higher temperature reduction zone where the carbonaceous constituents react with the iron oxide in the agglomerates to reduce the iron therein to metallic iron. Still further rotation of the hearth carries the largely reduced agglomerates into a high temperature fusion zone of the RHF where the iron melts and fuses to form iron nuggets and the slag fuses and separates from the metallic iron. Yet further rotation of the hearth carries the charge into the cooling zone of the furnace where both the iron and slag solidify. The hearth materials are then discharged for supplementary cooling and passivation.
One will recognize that in the production of DRI, the high temperature fusion and melting zone would not be included in the RHF. Rather, the solid DRI produced in the reduction zone would be cooled, discharged, and passivated.
The RHF has various inherent limitations. For example, feed distribution to the RHF is difficult because of the difference between the annular speed of the near and far sides of the hearth. Further, the feed must be pre-dried, i.e., if RHF area has to be dedicated to drying, the remainder of the RHF area available for production of DRI is reduced.
In addition, feedstock in the form of balls are considered a favored feedstock for iron ore concentrates to be used in a direct reduction process. Such balls are inherently fragile, especially when they contain nearly 40% volume of pulverized coal. Heat treatment of such balls in a RHF is generally non-uniform, i.e., balls on the short radius of the annular hearth receive intense direct radiation from wall burners for an appreciably greater length of time than those on the outer radius.
Further, discharge of such balls from the hearth requires that they maintain their physical integrity after reduction, which is often a problem. The balls are, for example, augered off the annular hearth and breakage could lead to jamming of the rotary hearth, damage to the hearth, or damage to an auger used for such discharge.
Various other limitations of the RHF relate to its physical construction. For example, the physical arrangement of a RHF necessarily leads to the cold feed side being next to the hot discharge side resulting in congestion and material handling complications. Further, the circular arrangement makes construction difficult (e.g., refractory, side walls, burners all have to be configured in a circular design) and the center of the RHF is congested and difficult to access for maintenance. Further, the design of the RHF, due to its circular arrangement, has size limitations placed thereon (e.g., about 60 meters diameter). For example, the hearth is generally massive and as such, problems in rotating such a large hearth increase with its size.
In addition to the RHF, other types of furnaces have also been described. For example, a paired straight hearth (PSH) furnace is described in U.S. Pat. No. 6,257,879B1 to Lu et al., issued Jul. 10, 2001, entitled “Paired straight hearth (PSH) furnaces for metal oxide reduction.”
The PSH furnace generally includes a pair of straight moving hearth furnaces located side by side, each having a charging end and a discharging end. Each furnace has a train of detachable hearth sections to enable each hearth section to be removed at the discharging end of one furnace and attached at the charging end of the other furnace. In other words, charge is moved by two straight hearth furnaces from one end to the other, i.e., two parallel solid flows in opposite directions using two side-by-side parallel furnaces. The first flow includes a first feed end, a paired furnace, and a first discharge end. The second flow includes a second feed end, a paired furnace, and a second discharge end. After the charge loaded in a hearth section at the feed end of each flow passes through one of the paired furnaces, the charge is discharged, and the hearth section is moved to the feed or charging end of the other flow to receive new charge.
However, the PSH furnace also has associated problems. For example, the charging end of one of the paired furnaces is right next to the discharging end of the other paired furnace. As such, there is no separation between the hot and cold ends of the paired furnaces. Further, in the PSH furnace, it is necessary to duplicate both charge delivery and product removal systems at each end of the furnace. This requires a complicated distribution system, or, for example, doubling the charge metering system for multiple components and the blending and drying systems.